I think that is what OP was fishing for. Wrong answers ;).

Llama.cpp is not harness, setting up prompt template to have instruct model is also not what people call harness.

What people call harness nowadays is software that you interact with and it proxies model interactions. It can do agentic loop but the loop is not really much use for models that do “thinking” unless you want to layer your own loop on top for dealing with results and good harness can help - but usually if something runs loop it is called an agent not a harness.

So if harness proxies all interaction with model it also provides tools for model. It can provide context management and security features, where as security features would be preventing injections and making sure model doesn’t access stuff it shouldn’t.